Premature development of knee osteoarthritis (OA) is recognized with increasing frequency following anterior cruciate ligament (ACL) injuries. ACL injury is one of the most common and debilitating athletic injuries in adolescents and young adults with a majority requiring an ACL reconstruction procedure. As early as 7-15 years following reconstruction, a large majority of individuals develop knee OA, a major cause of pain, functional disability and decline in quality of life throughout the adult life-span. The developmen of knee OA is a complex process resulting from multiple factors of which altered knee kinematics and kinetics are known contributors. Altered knee kinematics and kinetics are observed following ACL reconstruction during activities of daily living as well as during high-leve activities, and persist for as long as two years following ACL reconstruction. Our overall study objective is to investigate the role of knee kinematics and kinematics following ACL reconstruction on articular cartilage integrity and knee mechanics 5 years later. The first Aim wil determine if knee mechanics at post-ACL reconstruction are stable and track to 5 years later. The second Aim will determine if post-ACL reconstruction knee mechanics predict measures of cartilage integrity at 5 years following ACL reconstruction. For this study, we will utilize biomechanical and clinical data from 30 individuals that we previously collected following primary, unilateral ACL reconstruction. We will extend our previously collected data with magnetic resonance imaging and biomechanical data at 5 years post-ACL reconstruction in the same cohort. Three-dimensional knee kinematics and kinetics will quantify knee mechanics and 3-T MR imaging will be used to quantify morphological and compositional (T1-rho and T2 relaxation time) characteristics of knee articular cartilage. This approach will allow for immediat testing of our overall hypothesis that post-ACL reconstruction knee mechanics persist at 5 years later and are associated with measures of articular cartilage integrity. Through this proposal, we will establish the potential role of post-ACL reconstruction mechanics on early cartilage degeneration, advancing fundamental knowledge of the early pathogenesis of knee OA. Successful completion of this study will allow for the development and testing of rehabilitation and surgical treatment options for individuals with ACL injury that may better restore knee mechanics, and ultimately may mitigate development of premature OA.